1. Field of the Invention
The present invention pertains to a method for the regeneration of a particle filter arranged in the exhaust gas tract of an internal combustion engine and to a device for the regeneration of a particle filter arranged in the exhaust gas tract of an internal combustion engine. The invention pertains in particular to a method and to a device for regenerating particle filters in internal combustion engines operating with excess air.
2. Description of the Related Art
To minimize the fine particles, “particle separators” or particle filters are used in motor vehicles. A particle separator arrangement for motor vehicles is known from, for example, EP 10 727 65 A2. These particle separators differ from particle filters in that the exhaust gas stream is conducted along the separator structures, whereas, in the case of particle filters, the exhaust gas is forced to flow through the filter medium. As a result of this structural difference, particle filters tend to clog, which increases the exhaust gas backpressure. Particle filters cause an undesirable increase in the pressure at the exhaust gas outlet of the internal combustion engine, which in turn reduces engine power and leads to an increase in the amount of fuel consumed by the internal combustion engine. An example of a particle filter arrangement is known from EP 03 418 32 A2.
In both of the previously described arrangements, an oxidation catalyst located upstream of the particle separator or particle filter oxidizes nitrogen monoxide (NO) in the exhaust gas to nitrogen dioxide (NO2) with the help of the residual oxygen (O2) also present in the exhaust gas according to the following equation:2NO+O2<->2NO2 
In the particle filter, the NO2 reacts with the solid carbon-containing particles to form CO, CO2, NO2, and NO and thus regenerates the filter. The strong oxidizing agent NO2, makes it possible to achieve the continuous removal of the deposited fine particles know as passive regeneration. Nevertheless, this device and the way the method is implemented suffer from the disadvantage that a large amount of toxic NO2 is formed and/or is present in the exhaust gas system.
To prevent the escape of NO2 into the environment, care must therefore be taken to ensure that the area between the NO oxidation catalysts and the particle filters is sufficiently leak-proof. According to this method, not only NO2 but also SO3 is formed, the latter being produced on the platinum-containing NO oxidation catalysts from the sulfur in the fuel and/or motor oil. This SO3 and the NO2 condense on cold spots in the exhaust gas system and form highly corrosive sulfuric acid and nitric acid, so that the exhaust gas system must be made of high-grade steel up as far as the particle filter to avoid corrosion reliably.
It is also known that a particle filter can be regenerated by actively raising the exhaust gas temperature by supplying hydrocarbons (HC) and oxidizing them. For this purpose, DE 102 0050 552 40 A1, for example, describes a design in which an HC oxidation catalyst, a diesel particle filter, and then an SCR catalyst are arranged one after the other in the exhaust gas flow direction in the main exhaust gas train. A secondary exhaust gas train is also provided, which branches off from the main exhaust gas train upstream of the HC oxidation catalyst and which leads back into the main exhaust gas train after the diesel particle filter. A throttle for regulating the exhaust gas stream to be branched off, an oxidation catalyst, and a particle separator downstream of the oxidation catalyst are provided in the secondary exhaust gas train. In a design of this type, the throttle flap is kept closed during normal operation, so that all of the exhaust gas stream flows through the main exhaust gas train and is cleaned there. During a regeneration phase of the diesel particle filter in the main exhaust gas train, however, the throttle flap is opened to allow a portion of the exhaust gas stream to flow through the secondary exhaust gas train and thus bypass the diesel particle filter, after which the two exhaust gas streams, i.e., the stream flowing through the main exhaust gas train and the one flowing through the secondary exhaust gas train, are brought back together again at a mixing point upstream of the SCR catalyst.
As a result of this operating mode, the mass flow of exhaust gas through the diesel particle filter is decreased during the filter's regeneration phase, so that it is only necessary to raise the temperature of a smaller amount of exhaust gas, and the diesel particle filter can be regenerated with a smaller input of energy. In addition, by splitting the mass flow of exhaust gas into two parts and subsequently mixing the exhaust gas stream of the main exhaust gas train, which is at a high temperature, with the exhaust gas stream of the secondary exhaust gas train, which is at a low temperature, at the mixing point, the temperature of the exhaust gas stream flowing through the SCR catalyst can be reduced again. The particle separator in the secondary gas train, furthermore, prevents an exhaust gas stream from which soot particles have not been separated from leaving the exhaust gas train.
The hydrocarbons are added to the oxidation catalysts by an injection device directly upstream of the catalysts. Because, in a design of this type, the oxidation catalysts are oxidizing NO to NO2 even during non-regeneration mode, passive filter regeneration with NO2 takes place even in non-regeneration mode, although to only a small degree. This means that, NO2 is formed even during non-regeneration mode, and this is then usually emitted without being used. Because of the toxicity of NO2, this is impracticable and undesirable.
It is obvious that a design of this type has a relatively large number of parts, nor is it very compact, and thus overall it occupies a large amount of space.